As is known, epicyclic gearings are capable of transmitting motion between coaxial shafts, rotating at different speeds, and are very efficient in performing this function with limited weight and overall dimensions.
In some applications, reducing the overall outer dimension of the epicyclic gearing in radial direction, with respect to the axis of the two aforesaid shafts, is particularly important. For example, in the aeronautical propulsion field, in particular in the new engine architectures being studied to reduce consumption and pollution (such as “Geared Turbofan” and “Open Rotor” architectures), epicyclic gearings are used to transmit all of the power generated by the turbomachine to the propulsion system. Unlike the case, for example, of turbofan engines, in these applications the epicyclic gearing is integrated with the turbomachine, and therefore the diametral envelope thereof could influence the geometries of the channels for the passage of air flow or exhaust gases, and therefore penalise the output of the same turbomachine.
One of the essential elements in determining the dimensions of the epicyclic gearing in radial direction is constituted by the bearings of the planet gears. In particular, in the field of aeronautical engines, attention has recently been directed towards the replacement of rolling bearings with plain bearings or bushings, to couple the planet gears to the planet gear carrier or gear train carrier structure.
Prior art configurations in the field of aeronautical engines not only use planet gears supported by plain bearings, but also planet gears with double helical toothings. Solutions of this type have some critical points:                double helical toothings cannot be separated in their meshing, and therefore the gearing with its box must be mounted in the motor as a single component;        to mount the gearbox it is necessary to produce splined couplings with very small diameters which, as such, are subject to wear;        the architecture of the gear train carrier structure is relatively complex to minimise misalignments between the planet gears under load and sensitivity to construction errors;        plain bearings have a very low tolerance to contamination and can give rise to catastrophic and almost instant failures;        an auxiliary supply of lubricant is required to prevent damage to the plain bearings if the main lubrication system is not operating (i.e. while standing on the runway in the presence of wind or in case of failure), with consequent increase in weight and complexity of the engine.        
Other solutions maintain bearings of rolling type, but in order to reduce the dimensions of the planet gears they use two planet gear arrays, instead of one, arranged on opposite sides of an annular plate. In particular, the planet gears are mounted by means of the aforesaid rolling bearings on respective pins, which extend in cantilever fashion with respect to the plate and parallel to the axis of the gearing. A solution of this type is known, for example, from EP2339208A1, which corresponds to the preamble of claim 1.
The rotational torque is transmitted between the plate and a rotating transmission member by means of connection elements which are substantially parallel to the axis of the gearing, are generally called “tenons”, and are fixed with respect to the plate.
An example of this type of embodiment is visible in WO2002/079644.
In ideal conditions, this system balances the bending moments between the two arrays of planet gears and allows only a shear load to be relieved on the plate. However, in practice, the rigid connection between the tenons and the plate tends to make the plate bend during operation.
This bending has the effect of causing the axes of the pins that support the planet gears to bend, which leads to undesirable unbalancing between the loads on the planet gears between the two arrays, which gives rise to undesirable reaction stress in the connection area between the pins and the plate.
To overcome the effects caused by bending of the plate, its thickness could be increased. However, it is preferable not to exceed certain limits of thickness, as an axial dimension of the plate that is too large would make operation thereof particularly susceptible to construction errors (in particular to positioning errors of the planet gears, which again translate into significant overloads on the same planet gears).
The most widely used solution to reduce unbalance of the loads on the planet gears is the use of a configuration called “flex pin”, in which the pins supporting the planet gears have the ability to bend locally to automatically offset the effects of bending of the plate.
Unbalance of the loads on the planet gears can also be caused by radial deformations of the ring gear with which the planet gears mesh. In particular, the ring gear is constituted by a single part or by two half-rings fixed to each other and comprises an outer flange, one side of which is fixed to a transmission member for torque output.
Extraction of the torque on one side generates a radial deformation which is asymmetric with respect to the centreline plane. This deformation asymmetry leads to torque being transmitted to a greater extent on one, array of planet gears with respect to the other and therefore to one array of planet gears being subject to greater stress. This problem is normally solved by mounting an additional fin on the outer flange, on the opposite side with respect to that of the transmission member for motion output. However, this fin causes an increase in the number of components of the gearing and in the assembly times, and not always manages to optimise balancing of the torque transfer paths.